Pin 1 and 8 - 6L6/6CA7/EL34 Cathode Bias

[bryankloos]

Hey guys,

Just trying to confirm pin 1 and 8 in my build.
I'd like to be able to swap in 6L6/6CA7/EL34.
The amp is cathode biased. Do I connect pin 1 and 8 at the base or do I run pin 1 direct to its own ground.
With the former configuration, the suppressor grid will be at the same voltage of the cathode, and with the latter, it will be at ground, correct?
Please help me understand this.

Thanks,

Bryan

[Stevem]

Just jump pins 1 and 8 and then go to your cathode resistor.

Just note that the 6L6 family of Tubes biases in a good amount different then those others you will run so to get the best out of which ever you run you might want to use 2 separate cathode resistors and of course separate bypass caps that are selectable by means of a switch.

[thetragichero]

wouldn't you want pin 1 (supressor grid in el34, shield in metal-envelope 6l6) connected to ground rather than the cathode in a cathode biased amp?

[pdf64]

wouldn't you want pin 1 (supressor grid in el34, shield in metal-envelope 6l6) connected to ground rather than the cathode in a cathode biased amp?

I suppose it might be an option, but the cathode is the fundamental reference point for the other electrodes.
Here’s a Mullard cathode biased amp with the g3-k linked arrangement http://www.r-type.org/articles/art-003d.htm

Also the cathode voltage isn’t fixed, with a typical hot AB cathode biased amp it’s going to bounce around a bit according to signal level. So the g1-k and g2-k voltages will be changing with signal level, hence throwing g3-k into the mix too is yet another variable.

It may be interesting to try it

[sluckey]

wouldn't you want pin 1 (supressor grid in el34, shield in metal-envelope 6l6) connected to ground rather than the cathode in a cathode biased amp?

Matchless wasn't concerned...

https://el34world.com/charts/Schematics ... lubman.pdf
https://el34world.com/charts/Schematics ... ieftan.pdf

[thetragichero]

hmmm interesting. learn something everyday
I have heard of folks (maybe hifi folks?) tying the suppressor grid to a negative voltage (bias voltage) in a fixed bias amp

[Stevem]

How many folks actually have a very metal case 6L6 that they could run, I mean come on here!

The only reason I have one is that it's my calibration tube for tube tester!

[darryl_h]

Some Traynor guitar amplifiers had their EL34 suppressor grids connected to the negative grid bias, without any apparent ill effects. Opinions on the advantages of this procedure are mixed.

For additional information/opinion on suppressor grid machinations, primarily in the tube hifi ecosystem, search for "Hazen mod".

[TUBEDUDE]

The suppressor grid just repels the overly energetic electrons from bouncing off the plate. The more negative the voltage, the more effective the grid. If the more negative voltage varies, the grid ranges from effective to more effective. Since this grid is not a current carrier, unlike the screen grid, a moderately changing voltage should have no discernable effect.
I'd wire pin 1 to the raw bias if EL34's were used.
I'd not wire pins 1&8 separately if tetrodes could be used as pins 1&8 are internally connected in those and could result in the cathode being at less than ground potential.

[sluckey]

My 6L6s don't even have a pin 1.

[pdf64]

My understanding is that similarly to g1 and g2, as the voltage of g3 s is moved positive and negative (with respect to the cathode) from a notional operating point norm, anode current will increase and decrease.
Hence if g3 is moved from being connected to its cathode, to the (negative) bias supply, anode current will be reduced, and to restore anode current to its previous level, g1 or g2 would need to be made less negative / more positive.
However, I'm unclear how such a change would affect the anode curves, eg will the anode current on the g1 = 0V plot be reduced? As that might reduce the power output of the AB1 amps we use.
Referencing g3 to anything other than its cathode seems to be uncharted territory, literally

...
I'd not wire pins 1&8 separately if tetrodes could be used as pins 1&8 are internally connected in those and could result in the cathode being at less than ground potential.

I can't get my head around that, it doesn't seem correct or likely?
FYI by the late 50s, 6L6 etc were recognised as being pentodes, eg 'beam pentode'. Even prior to that, it was unusual for them to be described as 'beam tetrodes', rather 'beam power' valve seems to have been the term more commonly used by the manufacturers and in technical text.

[pompeiisneaks]

How many folks actually have a very metal case 6L6 that they could run, I mean come on here!

The only reason I have one is that it's my calibration tube for tube tester!

I have two metal case 6L6's somewhere in my stash, pulled them from an amp because I thought it was a cause of the problems, found out the problems and have forgotten to retry them in an amp, now that you've reminded me, I might try again!

~Phil

[TUBEDUDE]

My understanding is that similarly to g1 and g2, as the voltage of g3 s is moved positive and negative (with respect to the cathode) from a notional operating point norm, anode current will increase and decrease.
Hence if g3 is moved from being connected to its cathode, to the (negative) bias supply, anode current will be reduced, and to restore anode current to its previous level, g1 or g2 would need to be made less negative / more positive.
However, I'm unclear how such a change would affect the anode curves, eg will the anode current on the g1 = 0V plot be reduced? As that might reduce the power output of the AB1 amps we use.
Referencing g3 to anything other than its cathode seems to be uncharted territory, literally

...
I'd not wire pins 1&8 separately if tetrodes could be used as pins 1&8 are internally connected in those and could result in the cathode being at less than ground potential.

I can't get my head around that, it doesn't seem correct or likely?
FYI by the late 50s, 6L6 etc were recognised as being pentodes, eg 'beam pentode'. Even prior to that, it was unusual for them to be described as 'beam tetrodes', rather 'beam power' valve seems to have been the term more commonly used by the manufacturers and in technical text.

I hadn't experienced those effects of G3 controlling plate current to any degree. I'll experiment more thoroughly with my next EL34 build.
I know there are varied opinions on what a pentode is or isn't. Semantics aside, all I mean is if the cathode is internally connected to G3, in any tube that might be used there, wire 1&8 to the same place, ground. If just using tubes with a separate G3, wiring them separately will give more options. Like the early Canadian amps that wired G3 to the raw bias supply.